1. Field of the Invention
The present invention relates to a polyacetal resin composition having a significantly improved creep rupture life (creep characteristic) of a polyacetal resin in a metal insert molding and the like and a molded article thereof.
2. Description of the Related Art
A polyacetal resin has such excellent properties as mechanical properties, thermal properties, electric properties, slidability, moldability, and dimensional stability of a molding and is widely used as a structural material or a mechanism element in electrical apparatuses, automobile parts and precision instrument parts.
Meanwhile, it is known that although the polyacetal resin is less liable to cause creep than other thermoplastic resins, it is subjected to creep rupture depending on the form of an article molded therefrom and a usage pattern of the molding. For example, in the case of a metal insert molding using the polyacetal resin, due to stress caused by molding distortion at the time of molding or aftercontraction of the polyacetal resin after molding, cracks may occur in a portion when the stress converges in a very short time or even the product itself may be broken.
For this reason, in reality, use of the polyacetal resin in a metal insert molding such as a through anchor which is an automobile part is often impossible and is very limited even if it is actually used.
To improve such a creep rupture, measures with respect to design of the shape of a molding have heretofore been primarily taken. However, measures with respect to design of the shape are basically limited. Therefore, as a fundamental improvement measure, development of a resin material having a significantly improved creep rupture life is strongly desired.
As such a polyacetal resin material having an improved creep rupture life, JP-A 2000-265036 shows a resin composition obtained by mixing a silicone oil and an elastomer with a polyacetal resin. The resin composition has a significantly improved creep rupture life, as compared with conventional polyacetal resin materials. However, the resin composition will have to be further improved and meet higher demands.
The present inventors have made intensive studies so as to solve the above problems. As a result, they have found that a creep rupture life in a metal insert molding or the like is unexpectedly improved by mixing trace amounts of silicone oil, elastomer and ultra high molecular weight polyethylene into a high molecular weight polyacetal resin. The present invention has been completed by this finding.
That is, the present invention relates to a polyacetal resin composition prepared by mixing
(B) 0.05 to 3.0 wt % (in the composition) of silicone oil,
(C) 0.1 to 5.0 wt % (in the composition) of an elastomer, and
(D) 0.1 to 5.0 wt % (in the composition) of ultra high molecular weight polyethylene, into
(A) a polyacetal resin having a melt index of 3.0 or less, and to a metal insert molding using the composition.
In other words, the composition of the invention comprises (A), (B), (C) and (D) in the specified contents.